This invention relates to impact dot printers employed for printing alphanumeric characters on a printing medium and, more particularly, to printing logic for use in an impact printer having a carriage moveable across a printing medium in passes from side to side, motor drive means for selectably moving the carriage, a printhead including a plurality of vertically oriented printwires disposed for printing a row of draft quality characters in one pass at a first carriage speed or a row of letter quality characters including vertical strokes of a width requiring pairs of horizontally close adjacent dots in one pass at a second carriage speed which is about three times slower than the first carriage speed, printing medium advancing means for advancing the printing medium from one printing row position to a next printing row position, and printing logic connected to the motor drive means, for moving the carriage across the printing medium, the printhead, and the printing medium advancing means for causing the printhead to print dots on the printing medium with the printwires, and for advancing the printing medium from one printing row position to a next printing row position, the logic of the invention being adapted for increasing the throughput of the printer when printing letter quality characters and comprising logic for performing the steps of:
(a) deciding if the printer is to print draft quality or letter quality characters and if it is to print draft quality characters going to step (b), otherwise continuing to step (e);
(b) moving the carriage across the printing medium at the faster first carriage speed while printing all the dots of the characters comprising a row of characters;
(c) advancing the printing medium to a position for the printing of a next row of characters; and,
(d) if the printer is finished printing an indicated number of rows of characters to be printed, then exiting, otherwise returning to step (b);
(e) moving the carriage across the printing medium in a first pass at the faster first carriage speed while printing one of each pair of horizontally close adjacent dots included in the vertical strokes of the characters comprising a row of characters and a portion of other dots included in the row of characters;
(f) moving the carriage across the printing medium in a second pass at the same faster first carriage speed while printing the other of each pair of horizontally close adjacent dots included in the vertical strokes of the characters comprising the row of characters and any remaining unprinted other dots included in the row of characters;
(g) advancing the printing medium to a position for the printing of a next row of characters only after the first and second passes; and,
(h) if the printer is finished printing an indicated number of rows of characters to be printed, then exiting, otherwise returning to step (e).
As described in the parent application of which this is a continuation-in-part, U.S. Pat. No. 4,159,882 by Sanders et al. describes the first proposed multipass printing which, in some form, most of today's 9-pin printhead impact dot printers utilize. That patent teaches that by making two, three, or four passes with the printhead and with appropriate vertical motion between the printhead and the paper being printed on of, respectively, 1/2 pin spacing, 1/3 pin spacing or 1/4 pin spacing between passes, increased vertical resolution in the printed character could be obtained. This allowed the first Near Letter Quality (NLQ) and Letter Quality (LQ) characters to be printed by impact dot matrix printers. In such a system, there is no serious problem relative to increasing the horizontal resolution. As is readily known and appreciated by those skilled in the art, horizontal resolution (and character quality, including such features as bold typeface) can be created by simply doing more passes and/or partial vertical paper movement with a printhead having a relatively small number of pins.
As with most things involving computing, mechanical dot printing (involving for, example, impact, electrostatic, ink jet, etc. printheads) is a tradeoff between cost of materials (i.e. apparatus), print quality, and speed of throughput. In general, the impact type printhead have design limiting considerations with respect to speed of dot creation; that is, of the above-listed examples of dot creating printers, only the impact printer create their dots with a mechanical driving mechanism. In such printers, each dot is created by one end of a fine printwire being struck or pushed by a solenoid type actuator. The other end of the printwire is then pushed out of a printhead and strikes the printing medium through an inked ribbon which causes the ribbon at that point to create a dot on the printing medium. The limiting factor is the so-called "refire" rate, i.e. the time it takes for the mechanical printwire/actuator combination to strike one dot and then recycle to "fire" and create the next dot.
In an effort to improve the speed and quality of impact type printers, single pass printheads were introduced. Such a prior art printhead is depicted in simplified form in FIG. 1 where it is generally indicated as 10. The printhead 10 has a body containing two staggered rows of printwires 14. Other types of printwire arrangements are available; however, the principle of operation is the same as in the representative example of FIG. 1. The printheads are referred to by the number of printwires 14 (or "pins") they contain. Most often, such printheads contain eighteen or twenty-four pins. The 18-pin printheads are typically made with 11, 12 and 14 mil pins (i.e., printwires 14). The majority of 24-pin printheads are made with 8 mil pins as this type of printer was originally developed for Asian characters such as Kanji or Hanguel. These characters need the fine 10 mil strokes made by 8 mil pins because of their much greater character complexity. The 8 mil pins create a 10 mil dot on the print medium because of the intervening ribbon through which the printwire contacts the print medium, as described above.
When the above-described printheads are incorporated into printers intended for printing non-Asian characters as employed in the Western world, the stroke width must be increased. This, in turn, leads to serious print speed problems. Currently, 24-pin printers, such as the Toshiba model 321SL, print Letter Quality with one pass of the printhead at 6 inches per second (IPS) carriage speed, i.e. the transit speed at which the printhead 10 is moved laterally across the printing medium. The printer can print very good quality characters at 10 characters per inch (CPI), producing 60 characters per second (CPS). The dots are printed at 180 dots per inch (DPI) at this speed, although the horizontal resolution is 1/360". These printers are able to generate vertical strokes of 10, 15.5 or 21 mils width. The same quality letters could be printed at 120 DPI, which would increase the speed 50%, except that it would make the vertical strokes 10, 18.3 or 26.6 mils, i.e. either too thin or too thick for non-Asian characters.
The problem is depicted in FIGS. 3-5. As the printhead moves across the printing medium from, for example left to right as depicted in FIG. 3, the printwires 14 can only be fired after the refire time has elapsed since the last firing thereof. As depicted in FIG. 2, the vertical rows of 8 mil printwires 14 must be spaced from one another by integer multiples of the horizonal resolution. Thus, for example, the vertical rows of printwires 14 may be on 4/360" centers as depicted in the figure. This aspect is not critical to the problem being described, however. It is the refire time vis-a-vis the carriage speed that causes the problem. As depicted in the left portion of FIG. 5, the adjacent vertical rows of printwires 14 may be fired at slightly different times (depending on the horizontal spacing between the vertical rows) so as to fire along a common vertical line 16 and thereby create a vertical row 18 of over lapped dots 20 as depicted in the center portion of FIG. 5. As graphed in FIG. 4, given a fixed refire rate, the faster the printhead 10 traverses the printing medium, the greater the distance between the adjacent vertical lines 16 along which the dots 20 can be created. At a typical "draft speed", the dots 20 are spaced apart 1/16" between centers and 6.6 mils between dot edges as depicted in the center portion of FIG. 5. To get the LQ or NLQ character definition in one pass, the carriage speed must be drastically reduced to 1/3rd the draft speed giving a spacing between centers of 1/180" with a dot overlap of 2 mils as indicated in the graph of FIG. 4. This speed reduction is typically a factor of three, or even four; that is, in the NLQ or LQ mode, the printer operates at one-fourth to one-third the carriage speed that it employs in the draft mode.
Wherefore, it is the object of the present invention to provide a novel method of operating a 24-pin printhead, or the like, which will provide a substantial increase in the print speed of such printheads when printing in the NLQ or LQ mode.
Other objects and benefits of this invention will become apparent from the description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.